<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  
  <script>

    function fn() {
      let balance = Number(prompt('请输入银行卡余额'))
      let mealMoney = Number(prompt('输入当月食宿消费金额'))
      let expense = Number(prompt('输入当月生活消费金额'))

      balance = balance - mealMoney - expense

      return alert(`银行卡剩余金额${balance}`)

    }

    fn()
  </script>
</body>
</html>